chore(ci): run on "native" nix runner

.forgejo/workflows/update-flake.yml

@@ -6,7 +6,7 @@ on:
 
 jobs:
   update-flake:
-    runs-on: ubuntu-latest-amd64
+    runs-on: nix-amd64
 
     permissions:
       contents: write
@@ -15,12 +15,6 @@ jobs:
     - uses: actions/checkout@v4
       with:
         ref: master
-    - name: Install Nix
-      uses: https://github.com/DeterminateSystems/nix-installer-action@v4
-      with:
-        init: none
-        planner: linux
-        github-token: null
     - run: nix flake update
     - uses: https://github.com/stefanzweifel/git-auto-commit-action@v5
       with: